Rocking Curve Measurements Research Articles

Microstructure and influence of buffer layer on threading dislocations in (0 0 0 1) AlN/sapphire grown by hydride vapor phase epitaxy

The microstructures of AlN films grown by high-temperature hydride vapor phase epitaxy (HT-HVPE) on (0 0 0 1) sapphires with three different kind of buffer layers (BLs) were investigated by plan-view and cross-sectional transmission electron microscopy (TEM). The buffer layers were grown by HVPE at 1350 °C, MOCVD 1050 °C and HVPE at 1050 °C. The microstructures and in-plane misorientation of buffer layers are related to the crystal quality of the AlN films and dislocation evolution. The misorientation angles and average grain sizes of buffer layers were analyzed quantitatively using Moire fringe method. The buffer layer grown at 1350 °C by HVPE was recognized to be single crystalline, but buffer layers with mosaic-structure are formed at 1050 °C either by MOCVD or HVPE. High growth temperature of 1350 °C significantly reduced the in-plane misorientation of the buffer layer and the dislocation density was greatly reduced. AlN films on high-temperature growth buffer layers (∼1350 °C) have the lowest treading dislocation density ∼3 × 108 cm−2 determined using a combination of X-ray rocking curve measurement and plan-view transmission electron microscopy.

Crystal Phase Control of ε-Ga2O3 Fabricated using by Metal-Organic Chemical Vapor Deposition

e-Ga2O3 thin films were grown on (0001) sapphire, (0001) GaN, and a low-temperature buffer layer at different temperatures and flow rates of bubbled H2O (oxygen source) using trimethylgallium and H2O as precursors by using atmospheric-pressure metal-organic chemical vapor deposition. Due to the atmospheric pressure conditions, most of the Ga2O3 thin films were not grown in a pure e-phase, but contain a small portion of the β-phase. The crystal structure, crystal quality, phase ratio, and surface morphology were analyzed by using X-ray diffraction, rocking curve measurements, and field-emission scanning electron microscopy. A certain minimum H2O flow rate was required to form e-Ga2O3 thin films, and the optimal growth temperature for e-Ga2O3 was 650°C. The β-phase fraction of the mixed-phase (e + β) thin films was dominant at temperatures higher than 650 °C. The crystallinities and phase compositions of the thin films changed with the flow rate of H2O. Nearly single-crystalline Ga2O3 thin films were successfully grown on GaN and sapphire, but not on a low-temperature buffer layer. Hexagonally shaped Ga2O3 islands become aligned in an ordered direction that correlated to the substrate’s orientation during the initial stage of the growth and coalesced to complete a two-dimensional layer as the growth went on.

Studies on Lattice Strain Variation due to Nitrogen Doping by Synchrotron X-ray Contour Mapping Technique in PVT-Grown 4H-SiC Crystals

Lattice strain in 4H-SiC substrate wafers can have a deleterious effect on the performance of power electronic devices, especially under high-temperature operation. Significant strain can be introduced by lattice parameter change due to the incorporation of impurities in heavily doped 4H-SiC crystals. Synchrotron x-ray topographic contour mapping technique is able to deconvolute the lattice strain component from lattice tilt and thus generate strain maps, which has been incorporated into an anisotropic elasticity model to determine the nitrogen doping concentration in 4H-SiC substrate wafers. In order to further investigate the relationship between lattice strain and doping concentration, the lattice strain variation across the facet and off-facet regions in different 4H-SiC substrate wafers was studied. Hall effect measurements were carried out to measure the nitrogen concentration of 4H-SiC wafers, which shows a decrease in resistivity and Hall mobility with the increase of nitrogen concentration. The result shows that lattice strain within the basal plane is isotropic, while along the growth direction [0001], the strain value is one order magnitude lower. Qualitative study of lattice strain reveals more uniform distribution of strain inside the wafer facet compared to the outside regions. Additionally, wafers with higher nitrogen concentration were found to have larger overall lattice strain variation. Variation of lattice strain due to nitrogen doping was further confirmed by triple axis x-ray rocking curve measurements showing the highest full width at half maximum inside the wafer facet.

X-ray diffraction lineshape analysis of pulsed laser deposited ZnO nano-structured thin films

In this work, we report on the synthesis of highly textured c-oriented zinc oxide (ZnO) thin films on single crystal c-plane sapphire substrates using pulsed laser deposition (PLD). The growth experiments were performed at different substrate temperatures and oxygen pressures. The crystalline properties and surface morphology of the resulting films were studied using XRD peak analysis, rocking curve measurements, and SEM imaging. By varying deposition pressure and temperature, we observe different surface morphologies of the ZnO thin films, from smooth surfaces to clusters of grains and nanorods. The surface and volume structure of the films were found to be correlated. Within the range of experimental parameters used in this work, a growth temperature of 825 °C and a pressure of 10 mTorr lead to nearly epitaxial ZnO thin films having the largest grain size and the lowest strain perpendicular to the substrate surface i.e. along the growth axis.

Impact of thermal treatment on the growth of semipolar AlN on m-plane sapphire

The interest in semipolar orientations has been increasing because the reduced piezoelectric field can improve the performance of nitride-based optoelectronic devices. However, the crystalline quality of semipolar AlN on m-plane sapphire is still not good enough to realize light emitters with sufficiently high efficiency. We performed high-temperature annealing on AlN on m-plane sapphire to improve the crystalline quality. For (10-1-3) and (11-22) AlN on m-plane sapphire, the crystalline quality improved as the annealing temperature was increased up to 1700 °C, whereas beyond 1750 °C the AlN layer started to deteriorate and desorb. The crystalline quality was further improved by additional growth of AlN. In addition, X-ray rocking curve measurements and transmission electron microscopy confirmed that the density of stacking faults was reduced after the additional growth of AlN.

Laser molecular beam epitaxy growth of porous GaN nanocolumn and nanowall network on sapphire (0001) for high responsivity ultraviolet photodetectors

We report on high-responsivity metal-semiconductor-metal (MSM) structure based ultraviolet (UV) GaN photodetectors fabricated on various GaN nanostructures such as porous nanocolumn network (PNCN), nanowall networks (NWNs), and granular and compact thin films. Different GaN nanostructures were hetero-epitaxially grown on c-sapphire using laser molecular beam epitaxy by tuning the AlN buffer layer growth parameters. High resolution x-ray rocking curve measurements indicate that the crystalline quality of GaN critically depends on the selection of AlN buffer layer growth conditions such as type of ablation target and growth temperature. The porous GaN nanostructures revealed a nearly stress-free wurtzite structure as deduced by Raman spectroscopy measurements. Room temperature photoluminescence spectroscopy showed that the GaN films possess a near band emission (NBE) peak at ∼ 3.39 eV along with a broad yellow luminescence (YL) with maxima at 2.25 eV. For GaN PNCN and NWN structures, the NBE-to-YL emission ratio is more than an order higher compared to the GaN films. The fabricated MSM based UV-detector on GaN PNCN and NWN exhibited a high photo-responsivity of ∼27.72 and 24.8 A/W, respectively, under 2 V applied bias at room temperature. The GaN PNCN and NWN nanostructures with excellent photo-responsivity and optical quality prove to be promising candidates for the fabrication of efficient UV photodetectors due to their continuity in lateral direction, high surface area-to-volume ratio and tailored surfaces.

Evaluation of Model for Determining Nitrogen Doping Concentration from Resultant Strain in Heavily Doped 4H-SiC Crystals

Significant strain in 4H-SiC substrates can be introduced by impurity incorporation in heavily doped 4H-SiC crystals. The non-destructive X-ray double-crystal contour mapping method we reported recently is a powerful tool to estimate the lattice strain levels in 4H-SiC substrates with different doping concentrations. The lattice strain maps were derived from 11-20, 1-100 and 0008 reflections. Result shows that lattice strains within the basal plane and along the [0001] direction are in the order of 10-3 and 10-4 respectively. Hall effect measurements performed to determine the doping concentration of 4H-SiC substrates show an increase in doping concentration with the decrease of contact resistivity. X-ray rocking curve measurements were employed to confirm the existence of significant lattice distortion in heavily doped 4H-SiC substrates, which is in good agreement with the lattice strain analysis based on X-ray contour mapping method.

Crystallographic texture study of pulsed laser deposited Cr2O3 thin films

Chromium oxide thin films of thickness ~140 nm were grown on c-plane (0001) and a-plane (112¯0) sapphire substrates by Pulsed Laser Deposition (PLD) technique. The films of Cr2O3 on Si (100) substrates have been synthesized at an optimized oxygen partial pressure of 2 Pa with varying substrate temperature in the range of 473-973 K. It was observed that the films deposited at temperatures ≥673 K are crystalline as confirmed by X-Ray Diffraction (XRD) studies. The X-Ray Photoelectron Spectroscopy (XPS) profile showed that the films were grown with Cr3+ oxidation state. Atomic Force Microscopy (AFM) revealed smooth, uniform surface morphology with small amount of porosity. The epitaxial nature of the film on sapphire substrate and crystallographic orientation of the Cr2O3 over Si substrate were studied using rocking curve, ϕ (Phi) scan and pole figure measurement. Pole figure analysis suggested that the films deposited on Si substrate were fiber textured. Rocking curve Full Width at Half Maximum (FWHM) for Cr2O3 (0006) and Cr2O3 (112¯0) was about 0.1° and 0.06° respectively, which indicate the good quality of crystalline film. High Resolution X-ray Diffraction (HRXRD) with reciprocal space mappings (RSM) measurements indicate that the films grown on sapphire substrates are in relaxed state.

Progress on The Time-of-Flight Ultra Small Angle Neutron Scattering Instrument at SNS

We describe the Time-of-Flight (TOF) Ultra Small Angle Neutron Scattering (USANS) instrument at the Spallation Neutron Source, Oak Ridge National Laboratory (SNS, ORNL), which is now operating. The instrument, a double-crystal diffractometer (DCD), consists of a pair of triple-bounce symmetrically Bragg-reflecting Si(220) channel-cut crystals with a nominal Bragg angle of 70°, based on the Bonse-Hart multi-bounce concept. We briefly describe the instrument, present the results of measurements of double-crystal rocking curves, and exhibit a few early scientific results.

Influence of laser irradiation on optical properties of GaAsBi/GaAs quantum wells

This paper is focused on investigation of the impact of laser irradiation on the structural and optical properties of bismide-based multiple quantum wells (MQWs). The MQW structures, composed of 5 GaAsBi quantum wells, 7 nm thick, separated by 10 nm-thick GaAs barriers, were grown by molecular beam epitaxy on GaAs (100)-plane oriented semi-insulating substrates at 330°C temperature. The bismuth content in as-grown GaAsBi wells evaluated from the measurements of HR-XRD rocking curves was about 6%. HR-TEM and AFM studies of the MQWs evidenced sharp interfaces between the wells and barriers, and a smooth, droplet-free surface, respectively. HR-TEM images also evidenced a homogeneous bismuth distribution in the wells. The spatially-resolved photoluminescence study of GaAsBi/GaAs MQWs revealed the enhancement of PL emission efficiency of up to 80% with no shift of the spectral position after intense laser irradiation. The obtained results were explained by improvement of the GaAsBi crystal quality.

Effect of low NH3 flux towards high quality semi-polar (11-22) GaN on m-plane sapphire via MOCVD

The effect of ammonia flux towards the quality of the semi-polar (11-22) gallium nitride thin film on m-plane (10-10) sapphire is presented. Semi-polar (11-22) gallium nitride epi-layers were obtained using a two-step growth method, consisting of high temperature aluminum nitride followed by gallium nitride via metal organic chemical vapor deposition. The surface morphology analysis via field emission scanning electron microscopy and atomic force microscopy of the semi-polar (11-22) gallium nitride has shown that low ammonia flux promotes two-dimensional growth with low surface roughness of 4.08 nm. A dominant diffraction peak of (11-22) gallium nitride was also observed via X-ray diffraction upon utilizing low ammonia flux. The on- and off-axis X-ray rocking curve measurements illustrate the enhancement of the crystal quality, which might result from the reduction of the basal stacking faults and perfect dislocation. The full width half maximum values were reduced by at least 15% for both on- and off-axis measurements.

Optimizing growth, structure, and elastic-electrical properties of acentric melilite CaYAl3O7 crystal

A new melilite single crystal CaYAl3O7 with dimensions of Φ26 × 60 mm3 has been grown successfully by the Czochralski method, taking into consideration as a potential high-temperature piezoelectric crystal for the first time. The rocking curve measurements indicate that the as-obtained CaYAl3O7 crystal is of high quality with a full width at half-maximum (FWHM) of 42.1 arcsec from the (001) reflection. The complete set of room temperature dielectric, elastic, and piezoelectric constants of CYAM crystal were determined using resonance techniques and impedance analysis. Our results show the piezoelectric strain constants d14 and d36 are 12.6 and 2.18 pC/N, respectively, with the corresponding electromechanical coupling coefficients of k14 = 18.06% and k36 = 5.45%, indicating the crystal is a promising candidate for piezoelectric applications. The structural polyhedron distortions dipole moments of CYAM crystal were analyzed in detail, and the relationship between the crystal structure and the piezoelectric properties has also been discussed.

Growth of High‐Quality AlN and AlGaN Films on Sputtered AlN/Sapphire Templates via High‐Temperature Annealing

We investigated the fabrication and characterization of AlN and AlGaN epilayers grown on sputtered and annealed AlN/sapphire templates by metal organic vapor phase epitaxy. The surfaces of the AlN homoepilayers exhibited clear step‐terrace structures with minimal root mean square roughness values. X‐ray rocking curve (XRC) measurements showed that the AlN homoepilayers had smaller (102)‐plane full width at half maximum (FWHM) values (192 arcsec) than the AlN/sapphire templates did (214 arcsec), and that they had larger (0002)‐plane FWHM values (97 arcsec) than the AlN/sapphire templates (22 arcsec). The latter results may be due to the wafer curvature increasing as the AlN layer thickness increases. We also present results for the growth of AlGaN heteroepilayers with different AlN compositions on AlN/sapphire templates that either did or did not contain AlN regrowth layers. High‐AlN mole fraction AlGaN heteroepilayers had larger (0002)‐plane FWHM values and smaller (102)‐plane FWHM values than low‐AlN mole fraction AlGaN. Furthermore, the presence of an AlN regrowth layer tended to have a negative effect on the high‐AlN mole fraction AlGaN layers. The above results are discussed using lattice mismatch, wafer curvature, surface condition, and growth temperature parameters.

Growth and mechanical properties of near-stoichiometric LiNbO3 crystal

The near-stoichiometric LiNbO3 crystal (SLN) with dimension of Φ25 × 45 mm3 has been successfully grown from Li-rich solutions by the Czochralski method. The results of X-ray rocking curve measurement indicate that the as-grown SLN crystal has a relatively high crystallinity. The mechanical properties of SLN crystal were determined by nanoindentation method. The hardness and elastic modulus values obtained revealed higher values for (300) plane of SLN crystal (10.02 and 225.6 GPa) compared to the (600) plane of SLN crystal (7.09 and 202.8 GPa). Different hardness and elastic modulus values for different planes suggest anisotropic character of mechanical properties in SLN crystals. The distribution characteristic of residual stress in SLN crystal along the optical axis is analyzed by means of photoelasticity effect.

Polarity identification of ZnO(0001) surface by reflection high-energy electron diffraction

We measured rocking curves of reflection high-energy electron diffraction (RHEED) from ZnO(0001) polar surfaces. It was found that the rocking curves for each Zn- and O-terminated surface differ considerably. The relaxed structure of each polar surface was obtained from first-principles calculations using density functional theory. The results reveal compression relaxation of the top bilayer. Experimental rocking curves of each polar surface have been successfully reproduced by dynamical calculation of RHEED based on their relaxed surface structures. It was demonstrated that the RHEED method can identify the polarity of ZnO(0001) surfaces.

New insights into microstructural evolution of epitaxial Ni–Mn–Ga films on MgO (1 0 0) substrate by high-resolution X-ray diffraction and orientation imaging investigations

In this work, a detailed investigation has been performed on hetero-epitaxial growth and microstructural evolution in highly oriented Ni–Mn–Ga (1 0 0) films grown on MgO (1 0 0) substrate using high-resolution X-ray diffraction and orientation imaging microscopy. Mosaicity of the films has been analysed in terms of tilt angle, twist angle, lateral and vertical coherence length and threading dislocation densities by performing rocking curve measurements and reciprocal space mapping. Density of edge dislocations is found to be an order of magnitude higher than the density of screw dislocations, irrespective of film thickness. X-ray pole figure measurements have revealed an orientation relationship of || (1 0 0)MgO; || [0 0 1]MgO between the film and substrate. Microstructure predicted by X-ray diffraction is in agreement with that obtained from electron microscopy and atomic force microscopy. The evolution of microstructure in the film with increasing thickness has been explained vis-à-vis dislocation generation and growth mechanisms. Orientation imaging microscopy observations indicate evolutionary growth of film by overgrowth mechanism. Decrease in coercivity with film thickness has been explained as an interplay between stress field developed due to crystal defects and magnetic domain pinning due to surface roughness.

UNPASSIVATED HIGH OPERATING TEMPERATURE GaInAsSb INFRARED DETECTOR PHOTODETECTOR GROWN ON GaAs SUBSTRATE

Ga 0.87 In 0.13 As 0.4 Sb 0.96 photodiode structure was grown on semi-insulating 4” GaAs substrate by molecular beam epitaxy. The composition, crystal quality and dislocation density of epilayers were determined by high resolution X-ray diffraction rocking curve measurements. The threading dislocation density of the photodetector structure was calculated from the rotational broadening as ~2.5x10 8 cm -2 . The cutoff wavelength and the peak responsivity of the photodetector were determined as around 2.15 μm and 0.08 A/W at 300 K, respectively. By applying reverse bias (-100 mV) the responsivity value of the photodetector increases more than an order (~0.96 A/W) which is the best value reported up to now. Those results indicate that although there is a large lattice mismatch (~8.4%) between GaAs substrate and the photodetector structure, an acceptable photodetector performance was achieved which is important for reducing photodetector costs.

Bimorph Actuator: a New Instrument for Time-Resolved X-ray Diffraction and Spectroscopy

A new approach to time-resolved X-Ray experiments implementation at both laboratory X-Ray sources and synchrotron facilities is presented. A time resolution of few milliseconds per one rocking curve measurement can be reached by using a new X-Ray optics element – vibrating bimorph actuator based on a bidomain bending-type crystal of lithium niobate. The ranges of X-Ray beam angular or energy changeover are defined by the actuator operational mode and resonant response. The time and spatial parameters for X-ray optical element based on bimorph actuator have been estimated. The ranges of actuator angular changeover in a dynamical mode at various frequencies (including resonance) of bidomain crystal vibrations have reached up to 100 arcseconds in а quasistatic mode and more than 500 arcseconds in а resonance mode at the control voltage of 22 V and vibration frequency up to 8 kHz (which corresponds to time resolution of 0.125 milliseconds). The numerical algorithms for processing of the experimental data measured using the bimorph optical element have been developed. The examples of X-Ray beam angular changeover reached in different actuator vibration modes and possible application of such approach are presented in the work.

Crystal Growth and Linear and Nonlinear Optical Properties of KIO3·Te(OH)6

A single crystal of the nonlinear optical material KIO3·Te(OH)6 (KTI) with dimensions of 37 × 7 × 5 mm3 was successfully grown at room temperature by slow evaporation. Linear and nonlinear optical measurements were performed on these crystals. Rocking curve measurements indicate that the single crystal is of high quality with a full width at half-maximum (fwhm) of 0.023° (82 arcseconds) from the (010) reflection. Refractive index measurements revealed a birefringence of 0.0545 @ 1064 nm. Through the refractive index measurements and the fitted Sellmeier equations, we determined the Type I and Type II phase-matching wavelength ranges. KTI is Type I (Type II) phase-matchable with a fundamental wavelength range from 674–3266 nm (880–2260 nm). Powder second-harmonic generation (SHG) measurements indicate an SHG intensity of 1.2× KH2PO4 (KDP). SHG coefficients were determined by the Maker fringe method on cut and polished (100) and (010) single crystal wafers. The d31 = 0.24 pm/V and d32 = 0.73 pm/V were deter...

Crystal growth, structural, low temperature thermoluminescence and mechanical properties of cubic fluoroperovskite single crystal (LiBaF3)

Polycrystalline compounds of LiBaF3 were synthesized using conventional solid state reaction route and the phase purity was confirmed using powder X-ray diffraction technique. Using vertical Bridgman technique single crystal was grown from melt. Rocking curve measurements have been carried out to study the structural perfection of the grown crystal. The single peak of diffraction curve clearly reveals that the grown crystal was free from the structural grain boundaries. The low temperature thermoluminescence of the X-ray irradiated sample has been analyzed and found four distinguishable peaks having maximum temperatures at 18, 115, 133 and 216 K. Activation energy (E) and frequency factor (s) for the individual peaks have been studied using Peak shape method and the computerized curve fitting method combining with the Tmax- TStop procedure. Nanoindentation technique was employed to study the mechanical behaviour of the crystal. The indentation modulus and Vickers hardness of the grown crystal have values of 135.15 GPa and 680.81 respectively, under the maximum indentation load of 10 mN.